Unitary subcutaneous only implantable cardioverter-defibrillator and optional pacer

ABSTRACT

A unitary subcutaneous implantable cardioverter-defibrillator which has a long thin housing in the shape of a patient&#39;s rib. The housing contains a source of electrical energy, a capacitor, and operational circuitry that senses the presence of potentially fatal heart rhythms. Provided on the housing are cardioversion/defibrillation electrodes located to deliver electrical cardioversion-defibrillation energy when the operational circuitry senses a potentially fatal heart rhythm. The unitary subcutaneous implantable cardioverter-defibrillator does not have a transvenous, intracardiac, epicardial, or subcutaneous electrode.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/662,612, filed Sep. 15, 2003, now U.S. Pat. No. 7,289,854 and titled“Unitary Subcutaneous Only Implantable Cardioverter-Defibrillator andOptional Pacer”; which is a divisional of U.S. patent application Ser.No. 09/663,606, filed Sep. 18, 2000, now U.S. Pat. No. 6,647,292, andtitled “Unitary Subcutaneous Only Implantable Cardioverter-Defibrillatorand Optional Pacer”; the entire disclosures of which are incorporatedherein by reference.

This application is related to commonly owned U.S. patent applicationSer. No. 09/663,607, filed Sep. 18, 2000, now U.S. Pat. No. 6,721,597,and entitled “Subcutaneous Only Implantable Cardioverter-Defibrillatorand Optional Pacer”, the entire disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for performingelectrical cardioversion/defibrillation and optional pacing of the heartvia a totally subcutaneous non-transvenous system.

BACKGROUND OF THE INVENTION

Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias in the atriaand/or ventricles. Typically, electrodes are employed to stimulate theheart with electrical impulses or shocks of a magnitude substantiallygreater than pulses used in cardiac pacing.

Defibrillation/cardioversion systems include body implantable electrodesand are referred to as implantable cardioverter/defibrillators (ICDs).Such electrodes can be in the form of patches applied directly toepicardial tissue, or at the distal end regions of intravascularcatheters, inserted into a selected cardiac chamber. U.S. Pat. Nos.4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of whichare all incorporated herein by reference, disclose intravascular ortransvenous electrodes, employed either alone or in combination with anepicardial patch electrode. Compliant epicardial defibrillatorelectrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, thedisclosures of which are incorporated herein by reference. A sensingepicardial electrode configuration is disclosed in U.S. Pat. No.5,476,503, the disclosure of which is incorporated herein by reference.

In addition to epicardial and transvenous electrodes, subcutaneouselectrode systems have also been developed. For example, U.S. Pat. Nos.5,342,407 and 5,603,732, the disclosures of which are incorporatedherein by reference, teach the use of a pulse monitor/generatorsurgically implanted into the abdomen and subcutaneous electrodesimplanted in the thorax. This system is far more complicated to use thancurrent ICD systems using transvenous lead systems together with anactive can electrode, and therefore it has no practical use. It has, infact, never been used because of the surgical difficulty of applyingsuch a device (3 incisions), the impractical abdominal location of thegenerator and the electrically poor sensing and defibrillation aspectsof such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturersto produce ICDs which are small enough to be implanted in the pectoralregion. In addition, advances in circuit design have enabled the housingof the ICD to form a subcutaneous electrode. Some examples of ICDs inwhich the housing of the ICD serves as an optional additional electrodeare described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and5,658,321, the disclosures of which are incorporated herein byreference.

ICDs are now an established therapy for the management oflife-threatening cardiac rhythm disorders, primarily ventricularfibrillation (V-Fib). ICDs are very effective at treating V-Fib, but aretherapies that still require significant surgery.

As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, the requirement of ICD therapy touse intravenous catheters and transvenous leads is an impediment to verylong-term management as most individuals will begin to developcomplications related to lead system malfunction sometime in the 5-10year time frame, often earlier. In addition, chronic transvenous leadsystems, their implantation and removals, can damage majorcardiovascular venous systems and the tricuspid valve, as well as resultin life-threatening perforations of the great vessels and heart.Consequently, use of transvenous lead systems, despite their manyadvantages, are not without their chronic patient management limitationsin those with life expectancies of >5 years. Moreover, transvenous ICDsystems also increase cost and require specialized interventional roomsand equipment as well as special skill for insertion. These systems aretypically implanted by cardiac electrophysiologists who have had a greatdeal of extra training.

In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of automatic externaldefibrillator (AED) therapy. AEDs employ the use of cutaneous patchelectrodes to effect defibrillation under the direction of a bystanderuser who treats the patient suffering from V-Fib. AEDs can be aseffective as an ICD if applied to the victim promptly within 2 to 3minutes.

AED therapy has great appeal as a tool for diminishing the risk of deathin public venues such as in air flight. However, an AED must be used byanother individual, not the person suffering from the potentially fatalrhythm. It is more of a public health tool than a patient-specific toollike an ICD. Because >75% of cardiac arrests occur in the home, and overhalf occur in the bedroom, patients at risk of cardiac arrest are oftenalone or asleep and cannot be helped in time with an AED. Moreover, itssuccess depends to a reasonable degree on an acceptable level of skilland calm by the bystander user.

What is needed, therefore, is a combination of the two forms of therapywhich would provide prompt and near-certain defibrillation, like an ICD,but without the long-term adverse sequelae of a transvenous lead systemwhile simultaneously using most of the simpler and lower cost technologyof an AED. What is also needed is a cardioverter/defibrillator that isof simple design and can be comfortably implanted in a patient for manyyears. We call such a device a unitary subcutaneous only ICD (US-ICD)and is described in detail below.

SUMMARY OF THE INVENTION

The preferred embodiment for the unitary subcutaneous only ICD (US-ICD)with optional pacing consists of five basic components: 1) a curvedhousing which houses a battery supply, capacitor, and operationalcircuitry; 2) two cardioversion/defibrillating electrodes are attachedto the outer surface of the housing; 3) one or more sensing electrodeslocated on the housing; and 4) sense circuitry suitable to an ICD or AEDV-Fib detection algorithm. Additionally, an application system isprovided for simple insertion of the US-ICD. No transvenous lead systemis used, eliminating a significant impediment to broader scaleprophylactic use.

The housing will provide energy and voltage intermediate to thatavailable with ICDs and AEDs. The typical maximum voltage necessary forICDs using most biphasic waveforms is approximately 750 V and associatedmaximum energy of approximately 40 J. The typical maximum voltagenecessary for AEDs is approximately 2000-5000 V with an associatedmaximum energy of approximately 150-360 J. The US-ICD of the presentinvention will use voltages in the range of 800 to 2000 V and associatedwith energies of approximately 40-150 J.

The cardioversion/defibrillation electrodes are electrically insulatedfrom each other and are about 5-10 cm in length. In the preferredembodiment, the sense electrodes are located between thecardioversion/defibrillation electrodes and are spaced about 4 cm fromeach other to provide a reasonable QRS signal from a subcutaneousextracardiac sampling location but may be of variable length to allowfor sense optimization.

The sense circuitry in the preferred embodiment is designed to be highlysensitive and specific to the presence or absence of life-threateningventricular arrhythmias only. Features of the detection algorithm areprogrammable, but the algorithm is focused on the detection of V-Fib andhigh rate ventricular tachycardia (V-Tach) of greater than 240 bpm. Thistype of cardioverter-defibrillator is not necessarily designed toreplace ICD therapy for those with pre-identified problems ofV-Tach/V-Fib or even atrial fibrillation, but is particularly geared touse as a prophylactic, long-term device, used for the life of thepatient at risk of his/her first V-Fib/V-Tach event. The device of thepresent invention may infrequently be used for an actuallife-threatening event but can be employed in large populations ofindividuals at modest risk and with modest cost by physicians of limitedexperience. Consequently, the preferred embodiment of the presentinvention focuses only on the detection and therapy of the mostmalignant rhythm disorders. As part of the detection algorithm'sapplicability to children, the upper rate range is programmable upwardfor use in children, who are known to have more rapid supraventriculartachycardias as well as more rapid ventricular tachycardias compared toadults.

The incision to apply the device of the present invention can beanywhere on the thorax although in the preferred embodiment, the deviceof the present invention will be applied in the anterior mid-clavicularline approximately at the level of the mammary crease beneath the leftareolus. A subcutaneous path will then be made and will extend to theposterior thoracic region ideally at the level of the inferior scapulatip. Such a lead position will allow for a good transthoracic currentdelivery vector as well as positioning of the proximally positionedsense bipole in a good location for identification of the QRS ECGsignal. A specially designed curved introducer set, through which localanesthetic can be delivered, is provided to assist in the placement ofthe US-ICD.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is now made tothe drawings where like numerals represent similar objects throughoutthe figures where:

FIG. 1 is a schematic view of a Unitary Subcutaneous ICD (US-ICD) of thepresent invention;

FIG. 2 is a schematic view of the US-ICD subcutaneously implanted in thethorax of a patient;

FIG. 3 is a schematic view of the method of making a subcutaneous pathfrom the preferred incision for implanting the US-ICD;

FIG. 4 is a schematic view of an introducer for performing the method ofUS-ICD implantation; and

FIG. 5 is an exploded schematic view of an alternate embodiment of thepresent invention with a plug-in portion that contains operationalcircuitry and means for generating cardioversion/defibrillation shockwaves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, the US-ICD of the present invention isillustrated. The US-ICD consists of a curved housing 11 with a first andsecond end. The first end 13 is thicker than the second end 15. Thisthicker area houses a battery supply, capacitor and operationalcircuitry for the US-ICD. The circuitry will be able to monitor cardiacrhythms for tachycardia and fibrillation, and if detected, will initiatecharging the capacitor and then delivering cardioversion/defibrillationenergy through the two cardioversion/defibrillating electrodes 17 and 19located on the outer surface of the two ends of the housing. Examples ofsuch circuitry are described in U.S. Pat. Nos. 4,693,253 and 5,105,810,the entire disclosures of which are incorporated herein by reference.The circuitry can provide cardioversion/defibrillation energy indifferent types of waveforms. In the preferred embodiment, a 100 uFbiphasic waveform is used of approximately 10-20 ms total duration andwith the initial phase containing approximately ⅔ of the energy.However, any type of waveform can be utilized such as monophasic,biphasic, multiphasic or alternative waveforms as is known in the art.

In addition to providing cardioversion/defibrillation energy, thecircuitry can also provide transthoracic cardiac pacing energy. Theoptional circuitry will be able to monitor the heart for bradycardiaand/or tachycardia rhythms. Once a bradycardia or tachycardia rhythm isdetected, the circuitry can then deliver appropriate pacing energy atappropriate intervals through the electrodes. Pacing stimuli will bebiphasic in the preferred embodiment and similar in pulse amplitude tothat used for conventional transthoracic pacing.

This same circuitry can also be used to deliver low amplitude shocks onthe T-wave for induction of ventricular fibrillation for testing S-ICDperformance in treating V-Fib as is described in U.S. Pat. No.5,129,392, the entire disclosure of which is incorporated herein byreference. Also, the circuitry can be provided with rapid induction ofventricular fibrillation or ventricular tachycardia using rapidventricular pacing. Another optional way for inducing ventricularfibrillation would be to provide a continuous low voltage, i.e., about 3volts, across the heart during the entire cardiac cycle.

Another optional aspect of the present invention is that the operationalcircuitry can detect the presence of atrial fibrillation as described inOlson, W. et al., “Onset and Stability for Ventricular TachyarrhythmiaDetection in an Implantable Cardioverter and Defibrillator,” IEEEComputers in Cardiology, (1986) pp. 167-170, the disclosure of which isincorporated herein by reference. Detection can be provided via R-Rcycle length instability detection algorithms. Once atrial fibrillationhas been detected, the operational circuitry will then provide QRSsynchronized atrial defibrillation/cardioversion using the same shockenergy and waveshape characteristics used for ventriculardefibrillation/cardioversion.

The sensing circuitry will utilize the electronic signals generated fromthe heart and will primarily detect QRS waves. In one embodiment, thecircuitry will be programmed to detect only ventricular tachycardias orfibrillations. The detection circuitry will utilize in its most directform a rate detection algorithm that triggers charging of the capacitoronce the ventricular rate exceeds some predetermined level for a fixedperiod of time, for example, if the ventricular rate exceeds 240 bpm onaverage for more than 4 seconds. Once the capacitor is charged, aconfirmatory rhythm check would ensure that the rate persists for atleast another 1 second before discharge. Similarly, terminationalgorithms could be instituted that ensure that a rhythm less than 240bpm persisting for at least 4 seconds before the capacitor charge isdrained to an internal resistor. Detection, confirmation and terminationalgorithms as are described above and in the art can be modulated toincrease sensitivity and specificity by examining electrocardiographicQRS beat-to-beat uniformity, QRS signal frequency content, R-R intervalstability data, and signal amplitude characteristics, all or part ofwhich can be used to increase or decrease both sensitivity andspecificity of US-ICD arrhythmia detection function.

In addition to use of the sense circuitry for detection of V-Fib orV-Tach by examining the QRS waves, the sense circuitry can check for thepresence or the absence of respiration. The respiration rate can bedetected by monitoring the impedance across the thorax usingsubthreshold currents delivered across the active can and the highvoltage subcutaneous lead electrode and monitoring the frequency inundulation in the waveform that results from the undulations oftransthoracic impedance during the respiratory cycle. If there is noundulation, then the patient is not respiring and this lack ofrespiration can be used to confirm the QRS findings of cardiac arrest.The same technique can be used to provide information about therespiratory rate or estimate cardiac output as described in U.S. Pat.Nos. 6,095,987; 5,423,326; and 4,450,527, the entire disclosures ofwhich are incorporated herein by reference.

The housing of the present invention can be made out of titanium alloyor other presently preferred ICD designs. It is contemplated that thehousing is also made out of biocompatible plastic materials thatelectronically insulate the electrodes from each other. However, it iscontemplated that a malleable canister that can conform to the curvatureof the patient's chest will be preferred. In this way, the patient canhave a comfortable canister that conforms to the unique shape of thepatient's rib cage. Examples of conforming ICD housings are provided inU.S. Pat. No. 5,645,586, the entire disclosure of which is incorporatedherein by reference. In the preferred embodiment, the housing is curvedin the shape of a 5^(th) rib of a person. Because there are manydifferent sizes of people, the housing will come in differentincremental sizes to allow a good match between the size of the rib cageand the size of the US-ICD. The length of the US-ICD will range fromabout 15 to about 50 cm. Because of the primary preventative role of thetherapy and the need to reach energies over 40 Joules, a feature of thepreferred embodiment is that the charge time for the therapyintentionally be relatively long to allow capacitor charging within thelimitations of device size.

The thick end of the housing is currently needed to allow for theplacement of the battery supply, operational circuitry, and capacitors.It is contemplated that the thick end will be about 0.5 cm to about 2 cmwide, with about 1 cm being presently preferred. As microtechnologyadvances, the thickness of the housing will become smaller. Examples ofsmall ICD housings are disclosed in U.S. Pat. Nos. 5,957,956 and5,405,363, the entire disclosures of which are incorporated herein byreference.

The two cardioversion/defibrillation electrodes on the housing are usedfor delivering the high voltage cardioversion/defibrillation energyacross the heart. In the preferred embodiment, thecardioversion/defibrillation electrodes are coil electrodes. However,other cardioversion/defibrillation electrodes could be used such ashaving electrically isolated active surfaces or platinum alloyelectrodes. The coil cardioversion/defibrillation electrodes are about5-10 cm in length. Located on the housing between the twocardioversion/defibrillation electrodes are two sense electrodes 25 and27. The sense electrodes are spaced far enough apart to be able to havegood QRS detection. This spacing can range from 1 to 10 cm with 4 cmbeing presently preferred. The electrodes may or may not becircumferential with the preferred embodiment. Having the electrodesnon-circumferential and positioned outward, toward the skin surface, isa means to minimize muscle artifact and enhance QRS signal quality. Thesensing electrodes are electrically isolated from thecardioversion/defibrillation electrode via insulating areas 23.Analogous types of cardioversion/defibrillation electrodes are currentlycommercially available in a transvenous configuration. For example, U.S.Pat. No. 5,534,022, the entire disclosure of which is incorporatedherein by reference, discloses a composite electrode with a coilcardioversion/defibrillation electrode and sense electrodes.Modifications to this arrangement are contemplated within the scope ofthe invention. One such modification is to have the sense electrodes atthe two ends of the housing and have the cardioversion/defibrillationelectrodes located in between the sense electrodes. Another modificationis to have three or more sense electrodes spaced throughout the housingand allow for the selection of the two best sensing electrodes. If threeor more sensing electrodes are used, then the ability to change whichelectrodes are used for sensing would be a programmable feature of theUS-ICD to adapt to changes in the patient physiology and size over time.The programming could be done via the use of physical switches on thecanister, or as presently preferred, via the use of a programming wandor via a wireless connection to program the circuitry within thecanister.

The housing will provide energy and voltage intermediate to thatavailable with ICDs and most AEDs. The typical maximum voltage necessaryfor ICDs using most biphasic waveforms is approximately 750 volts withan associated maximum energy of approximately 40. Joules. The typicalmaximum voltage necessary for AEDs is approximately 2000-5000 volts withan associated maximum energy of approximately 200-360 Joules dependingupon the model and waveform used. The US-ICD of the present inventionuses maximum voltages in the range of about 800 to about 2000 volts andis associated with energies of about 40 to about 150 Joules. Thecapacitance of the S-ICD could range from about 50 to about 200 microfarads.

The sense circuitry contained within the housing is highly sensitive andspecific for the presence or absence of life-threatening ventriculararrhythmias. Features of the detection algorithm are programmable, andthe algorithm is focused on the detection of V-Fib and high rate V-Tach(>240 bpm). Although the US-ICD of the present invention may rarely beused for an actual life-threatening event, the simplicity of design andimplementation allows it to be employed in large populations of patientsat modest risk with modest cost by non-cardiac electrophysiologists.Consequently, the US-ICD of the present invention focuses mostly on thedetection and therapy of the most malignant rhythm disorders. As part ofthe detection algorithm's applicability to varying patient populations,the detection rate range is programmable upward or downward to meet theneeds of the particular patient based on their cardiac condition andage.

Turning now to FIG. 2, the optimal subcutaneous placement of the US-ICDof the present invention is illustrated. As would be evident to a personskilled in the art, the actual location of the US-ICD is in asubcutaneous space that is developed during the implantation process.The heart is not exposed during this process and the heart isschematically illustrated in the figures only for help in understandingwhere the device and its various electrodes are three-dimensionallylocated in the thorax of the patient. The US-ICD is located between theleft mid-clavicular line approximately at the level of the inframammarycrease at approximately the 5^(th) rib and the posterior axillary line,ideally just lateral to the left scapula. This way the US-ICD provides areasonably good pathway for current delivery to the majority of theventricular myocardium.

FIG. 3 schematically illustrates the method for implanting the US-ICD ofthe present invention. An incision 31 is made in the left anterioraxillary line approximately at the level of the cardiac apex. Asubcutaneous pathway is then created that extends posteriorly to allowplacement of the US-ICD. The incision can be anywhere on the thoraxdeemed reasonable by the implanting physician although in the preferredembodiment, the US-ICD of the present invention will be applied in thisregion. The subcutaneous pathway is created medially to the inframammarycrease and extends posteriorly to the left posterior axillary line. Thepathway is developed with a specially designed curved introducer 42 (seeFIG. 4). The trocar has a proximal handle 41 and a curved shaft 43. Thedistal end 45 of the trocar is tapered to allow for dissection of asubcutaneous pathway in the patient. Preferably, the trocar iscannulated having a central lumen 46 and terminating in an opening 48 atthe distal end. Local anesthetic such as lidocaine can be delivered, ifnecessary, through the lumen or through a curved and elongated needledesigned to anesthetize the path to be used for trocar insertion shouldgeneral anesthesia not be employed. Once the subcutaneous pathway isdeveloped, the US-ICD is implanted in the subcutaneous space, and theskin incision is closed using standard techniques.

As described previously, the US-ICDs of the present invention vary inlength and curvature. The US-ICDs are provided in incremental sizes forsubcutaneous implantation in different sized patients. Turning now toFIG. 5, a different embodiment is schematically illustrated in explodedview which provides different sized US-ICDs that are easier tomanufacture. The different sized US-ICDs will all have the same sizedand shaped thick end 13. The thick end is hollow inside allowing for theinsertion of a core operational member 53. The core member comprises ahousing 57 which contains the battery supply, capacitor and operationalcircuitry for the US-ICD. The proximal end of the core member has aplurality of electronic plug connectors. Plug connectors 61 and 63 areelectronically connected to the sense electrodes via pressure fitconnectors (not illustrated) inside the thick end which are standard inthe art. Plug connectors 65 and 67 are also electronically connected tothe cardioverter/defibrillator electrodes via pressure fit connectorsinside the thick end. The distal end of the core member comprises an endcap 55, and a ribbed fitting 59 which creates a water-tight seal whenthe core member is inserted into opening 51 of the thick end of theUS-ICD.

The core member of the different sized and shaped US-ICD will all be thesame size and shape. That way, during an implantation procedure,multiple sized US-ICDs can be available for implantation, each onewithout a core member. Once the implantation procedure is beingperformed, then the correct sized US-ICD can be selected and the coremember can be inserted into the US-ICD and then programmed as describedabove. Another advantage of this configuration is when the batterywithin the core member needs replacing it can be done without removingthe entire US-ICD.

The US-ICD device and method of the present invention may be embodied inother specific forms without departing from the teachings or essentialcharacteristics of the invention. The described embodiments aretherefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description, and all changes whichcome within the meaning and range of equivalency of the claims aretherefore to be embraced therein.

The invention claimed is:
 1. A method of implanting a cardiac stimulusdevice into a patient comprising: developing a space for receiving thecardiac stimulus device beneath the skin and over the ribcage of thepatient; and inserting a unitary cardiac stimulus device into thedeveloped space; wherein: the unitary cardiac stimulus device comprisespower supply, electronics, and electrodes such that, once implantationis complete, each of the power supply, electronics and electrodes aredisposed within the developed space between the skin and the ribcage ofthe patient; the unitary cardiac stimulus device comprises an elongatedhousing that has an elongated shape that conforms to the shape of one ofthe patient's ribs once implanted; the unitary cardiac stimulus devicelacks any transvenous leads; all of the electrodes of the unitarycardiac stimulus device are disposed on the housing, such that theimplantation is completed without inserting any transvenous leads intothe patient; and the step of inserting the unitary cardiac stimulusdevice into the developed space does not include accessing the patient'svenous system.
 2. The method of claim 1 wherein the developed spaceextends between the posterior axillary line and the anteriormid-clavicular line of the patient.
 3. The method of claim 1 wherein theelongated housing has an elongated curve, the elongated curve of thehousing sized to extend from an anterior region to a posterior region ofthe patient when implanted into the developed space.
 4. The method ofclaim 1 wherein the unitary cardiac stimulus device comprises an outerhousing that contains and protects the electronics, wherein theelectrodes are disposed on the housing such that, once implanted in thedeveloped space, at least a first electrode is disposed in a posteriorregion of the patient and a second electrode is disposed in an anteriorregion of the patient, with a portion of the patient's heart disposedtherebetween.
 5. The method of claim 1 wherein the unitary cardiacstimulus device includes at least three electrodes placed at spacedlocations thereon; and the method further comprises programming theunitary cardiac stimulus device, once implanted, to use two best sensingelectrodes in analyzing cardiac activity; and adapting the selection oftwo best sensing electrodes to changes in patient physiology over time.6. The method of claim 5 wherein the unitary cardiac stimulus devicecomprises a canister having the electrodes thereon, the canister furthercomprising physical switches disposed thereon, and the step ofprogramming the unitary cardiac stimulus device is done via the switcheson the canister.
 7. The method of claim 5 wherein the step ofprogramming the unitary cardiac stimulus device is performed using aprogramming wand.
 8. The method of claim 5 wherein the step ofprogramming the unitary cardiac stimulus device is performed via awireless connection.
 9. The method of claim 5 wherein the space forimplanting extends between the anterior mid-clavicular line and theposterior axillary line; and the step of inserting the unitary cardiacstimulus device is performed such that a first electrode is disposednear an end of the unitary cardiac stimulus device in a posterior regionof the patient; second and third electrodes are disposed in an anteriorregion of the patient; and a fourth electrode is disposed near an end ofthe unitary cardiac stimulus device in an anterior region of thepatient.
 10. The method of claim 9 wherein the first and fourthelectrodes are coupled to internal circuitry within the unitary cardiacstimulus device as defibrillation electrodes, and the second and thirdelectrodes are coupled to the internal circuitry as sensing electrodes.11. The method of claim 9 wherein the first and fourth electrodes arecoupled to internal circuitry within the subcutaneous cardiac stimulusdevice as sensing electrodes, and the second and third electrodes arecoupled to the internal circuitry as defibrillation electrodes.
 12. Themethod of claim 5 wherein the unitary cardiac stimulus device has anelongated shape and generally conforms to one of the patient's ribs onceimplanted into the patient.